(1) Field of the Invention
The present invention concerns a system for controlling hydraulic pressure in an automatic four-speed transmission.
(2) Description of the Prior Art
The automatic transmission comprises a torque converter, multi-stage gear shift mechanism connected with the torque converter, and a plurality of friction elements for selecting one of the gear shift stages. The friction elements are worked through a plurality of control valves for controlling the hydraulic pressure applied by the hydraulic control system.
Such a hydraulic control system comprises a pressure regulating means for regulating the hydraulic pressure produced by an oil pump, manual and automatic transmission control means for selecting one of the shift modes, pressure control means for controlling the shifting sense and responsiveness to make a gear shift smooth, damper clutch control means for working the damper clutch of the torque converter, and pressure distribution means for distributing properly the hydraulic pressure among the friction elements. The pressure control means controls the pressures applied to the friction elements, torque converter, solenoid valves, etc., which substantially affect the shifting sense.
When performing the shift operation, it is preferable to supply controlled hydraulic pressures to the friction elements while completing the shift operation with the drive pressure. Such transmission control means are disclosed in U.S. patent application Ser. No. 08/347,312, now U.S. Pat. No. 5,507,706, filed by the present applicant, which suffers the shift shock caused by the drive pressure or the backward pressure applied to the friction elements in the manual shifting when making the forward or the backward movement. When this means is applied to an automatic transmission of a vehicle providing four gearshift stages, the engine brake is not worked in the "D" range third speed, so that, if the engine brake is not worked in the emergency mode of the third speed, a dangerous situation is encountered due to the vehicle's running inertia. Moreover, if the second to third one-way clutch is omitted, the skip shift control from the fourth speed to the second speed can not be performed smoothly, thus resulting in the occurrence of a shift shock.